我的Lambda表达有什么问题？

Question

I'm trying to write a simple Lambda expression in C#:

int numElements = 3;
string[]firstnames = {"Dave", "Jim", "Rob"};
string[]lastnames = {"Davidson", "Jameson", "Robertson"};

List<Person> people = new List<Person>();

for(int i = 0 ; i < numElements; i++)
{
    people.Add(new Person { FirstName = firstnames[i], LastName = lastnames[i] });                
}

bool test = people.Contains(p => p.FirstName == "Bob");

My understanding of Lambda expressions and how they work is still a little shady and I miffed as to why this will not work...I'm trying to find out if a list contains a name...

Answer 1

Are you looking for:

bool test = people.Any(p => p.FirstName == "Bob");

Or are you mixing Rob and Bob?

Answer 2

The problem here is not lambdas but instead the boundaries of the for loop. The arrays you defined have a length of 3 but numElements is defined to have a value of 10 . This means you will get an exception for an illegal array index on the 4th iteration of the loop. Try the following

int numElements = 3;

Or more simply remove the numElements variable and instead iterate to the length of the firstnames array

for (int i = 0; i < firstnames.length; i++) {
  ...
}

EDIT

OP indicated that the numElements originally posted was a typo. Other possible sources of error in the code

• Use "Rob" instead of "Bob" if you want to find a matching element
• The Contains method on GenericList<T> needs to have a compatible delegate signature. Func<T, bool> for example

Answer 3

1. Make sure you are linking the System.Linq namemespace, ie

    using System.Linq; 

2. You are using the Contains method. This method expects a Person and will use an equality comparison to determine if your collection already contains it. In the default case, the equality comparison defaults to reference comparison so it will never contain it, but that's another topic.

3. To achieve your goal, use the Any method. This will tell you if ANY of the elements in your collection conform to a condition.

    people.Any(p => p.FirstName == "BoB"); 

You may want to read about the extension methods First and FirstOrDefault and Where as they would also solve your problem.

Answer 4

You don't set numElements to the correct value ( you set it to 10, but your arrays only have 3 values) - furthermore you don't even need it, just use a collection initializer instead of those separate string arrays:

GenericList<Person> people = new GenericList<Person>()
{
    new Person { FirstName = "Dave", LastName = "Davidson" },
    new Person { FirstName = "Jim", LastName = "Jameson" }
    new Person { FirstName = "Rob", LastName = "Robertson" }
}

Now assuming your GenericList<T> class implements IEnumerable<T> you can use Any() to do your test:

bool test = people.Any(p => p.FirstName == "Bob");

Answer 5

what your real problem with this lambdas ?

If because you have test false then that's true you don't have "Bob" in firstName

bool test = people.Contains(p => p.FirstName == "Bob");

and

string[]firstnames = {"Dave", "Jim", "Rob"};

Answer 6

Couple of problems here.

One: GenericList is not a type. You were probably looking for the generic type System.Collections.Generic.List<T> .

Two: Contains accepts a Person in your example, not a delegate (lambdas are a new way to write delegates as of C# 3). One way to get what you want here would be to combine Where and Count , in the form of bool test = people.Where(p => p.FirstName == "Bob").Count() > 0;

Answer 7

// We will use these things:
Predicate<Person> yourPredicate = p => p.FirstName == "Bob";
Func<Person, bool> yourPredicateFunction = p => p.FirstName == "Bob";
Person specificPerson = people[0];

// Checking existence of someone:
bool test = people.Contains(specificPerson);
bool anyBobExistsHere = people.Exists(yourPredicate);

// Accessing to a person/people:
IEnumerable<Person> allBobs = people.Where(yourPredicateFunction);
Person firstBob = people.Find(yourPredicate);
Person lastBob = people.FindLast(yourPredicate);

// Finding index of a person
int indexOfFirstBob = people.FindIndex(yourPredicate);
int indexOfLastBob = people.FindLastIndex(yourPredicate);

You should play with LINQ methods somewhile...